1. Field of the Invention
The present invention relates to a facsimile (hereinafter called fax) transmission method on Code Division Multiple Access (hereinafter called CDMA) cellular networks. Particularly, It provides a reliable fax service on CDMA mobile communication system using a general computer networking environment being able to access the Internet.
2. Description of the Conventional Art
Group III (hereinafter called G3) fax is one of the fax standards recommended by the ITU-T(previously CCITT), by which it processes an image signal digitally and has a large amount of information because it increases the number of pixels for obtaining the same good resolution as that of an analog signal.
The said G3 fax service is implemented based on the ITU-T recommendation T.30 for a general switched telephone network including a fax calling identification, a parameter negotiation, a page transmission, a receiving identification, and a calling release, etc, and the ITU-T recommendation T.4 protocol for a fax machine for encoding an image signal of a document.
FIG. 1 is a brief block diagram illustrating a group III fax service system for a conventional public switched telephone network (hereinafter called PSTN), which has a transmission unit 100 for a calling part, the PSTN 200 for transmitting a fax signal and a reception unit 300 for a called part. The transmission unit 100 includes a CCD scanner 110 for reading an image data to an analog signal, an analog/digital (A/D) converter 120 for converting the analog signal from the CCD scanner 100 into a digital signal, MH/MM/MMR compression unit 130 for compressing the converted digital signal following the T.4 protocol, and a fax modem 140 for converting the compressed digital signal into the analog signal for transmitting through the PSTN 200. The reception unit 300 includes an fax modem 310 for converting the analog signal receiving from the PSTN 200 into digital signal for signal processing, MH/MR/MMR expansion 320 for decompressing the signal following the T.4 protocol and a thermal printer 330 for printing the image signal.
The fax service on PSTN 200 is implemented on a fax apparatus with the transmission unit lOC and the reception unit 300 or a personal computer with fax application software and the fax modems 140 and 310. The fax apparatus or the personal computer can be called as a fax terminal.
The modems 140 and 310 follow the transmission procedure of the T.30 and they are used for the end-to-end data conversion.
However, in CDMA cellular networks, a data conversion is required only when a CDMA cellular network is interworked with a non-CDMA cellular network such as a PSTN or an analog cellular network because the digital image data or a fax terminal can be delivered digitally between the fax terminal and the network.
Therefore, there is an interworking function (hereinafter called IWF) in a fixed station such as a base station or a mobile switching unit for the said data conversion in the CDMA cellular network. The IWF has fax modem pools and protocol conversion functions and speed matching functions between sub-systems of the networks.
Since the T.30 and the T.4 protocol for GIII fax service were proposed for error-free wired network such as general switched telephone network, the using of the protocols in the CDMA cellular network which has high Error and call drop by signal noise, fading and frequent handover, may cause a degradation of the service.
So the CDMA cellular networks have a protocol stack for compensating the service degradation on the unreliable CDMA radio channel. The protocol stack is shown in FIG. 2 and it is recommended as a standard in IS-99 which is an EIA/TIA standard for asynchronous data and fac service for wideband spread spectrum.
The asynchronous data 401 and thee fax 402 represent an application software for an asynchronous data and a fax service, the application interface 403 processes the standard/expanding AT modem commands and negotiates a data compression.
This protocol stack is based on the transport control protocol 404/internet protocol 406(hereinafter called TCP/IP) which is popularly used for the Internet service. The internet control message protocol (hereinafter called ICMP)405 handles a control message for IP. The internet protocol control protocol (hereinafter called IPCP) 408, the Link control protocol (hereinafter called LCP) 409, and the point-to-point protocol (hereinafter called PPP) 410 are used for transmitting the TCP/IP packet through the low rate serial link. The sub-network dependent convergence function (hereinafter called SNDCF) 407 is used for a TCP/IP header compression. Hereinafter, the said application software 401/402, the said application interface 403, the TCP/IP 404/406, the ICMP 405, the IPCP 408, the LCP 409, the PPP 410 and the SNDCF 407 are used as upper layer protocols.
In addition, the radio link protocol (hereinafter called RLP) 411 is used for a link layer protocol for providing an octet stream transport service over radio channel. The RLP layer segments a variable length data packet of an upper layer into a CDMA traffic channel frame. It operates on a featureless octet stream and delivers the octet in the order received from the upper layer because there is no direct relationship between the upper layer packet and CDMA traffic channel frames.
FIG. 3 shows the example of a fax transmitting/receiving procedure on a conventional PSTN for illustrating the ordinary fax service procedure in the conventional art and comparing with one of the CDMA cellular network.
The calling part 510 and called part 520 of FIG. 3 are configured with a data terminal equipment (hereinafter DTE) 510a/520b and a data circuit equipment (hereinafter called DCE) 510b/520a and use a G3 fax class II modem command.
Though the modem command for controlling the fax modem can differ with each modem manufacturer, the operating methods thereof is implemented base on the ITU-T recommendation T.30 procedure. Since all fax terminal do not follow the T.30 fully, the procedure showing FIG. 3 may also have some difference with the standards.
The said ITI-T recommendation T.30 has five separate and consecutive phases A, B, C, D and E. A phase A establishes the fax call, a phase B is a pre-message procedure for identifying and selecting the required facilities, a phase C is a message transmission procedure, a phase D is a post-message procedure including an end-of-message and a confirmation and multi-document procedures, and a step E releases the call.
These phases represent the call establishment and release based on the common rules used on general switched telephone network and the fax transmission identification, inspection and control.
Following the FIG. 3, when the DTE 510a of the calling part initiates a call using an modem dial command (ATD) with dial number, the DEC 510b of the calling part sends a dial tone to the DCE 520a of the called part and detects the call progress in phase A. Thereafter it generates a CIIG tone and waits for a DIS tone from the DCE 520a of the called part.
When the DCE 510b detects the V.21 Ch.2 modulated by 300 bits/s HDLC flags, it sends a confirmation response message (+FCON) informing the DTE 510a of the normal connection with the called part.
In the phase B, the DCE 510b of the calling part and DCE 520a of the called part identify their capabilities such as a vertical resolution and bit rate, etc, and their appropriate receiving condition, etc, with a received function identification tone (DIS). Then the DCE 510b of the calling part generates a receiving command tone(DCS) continuously based on the DIS tone and current session parameter (+FDIS) fixed before. When the DTE 510a of the calling part sends the data transmission acknowledgement message (+FDT), the DCE 510b of the calling part stops the DCS tone and the phase C is going on.
Entering the phase C, both part transmit and receive the actual image data and control character &lt;DLE&gt;&lt;ETX&gt;.
When the one page document is transmitted, the post-message procedure including end-of-message and multi-document procedure in the phase D is progressed. In the phase D, if DCE 510b receives the previous page result response (+FET) with value 0 which means the next page documents is remain from DTE 510a, the phase B, C, and D are repeated until a previous page result response (+FET) will be value 2 which means the end of page.
Then, in the phase E, the call is released and the DCE 510b of the calling part and DCE 520a of the called part transmit the modem hangup code (+FHND) with hangup code to each DTE 510a of the calling part and DTE 520b of the called part.
In order to implement the fax services on the CDMA cellular networks, the combination and synchronization of the serial call processing procedure between the mobile station and the base station should be combined with the fax transmission procedure of PSTN based on the ITU-T recommendation T.30 shown in the FIG. 3 and the procedures should be synchronized.